Symposium EP13 : Tailoring Superconductors---Materials and Devices from Basic Science to Applications

Whenever novel classes of unconventional superconductors such as the iron-arsenides are discovered, initial research struggles with the complications arising from the often complex chemistry. The first samples rarely are high quality single crystals, but powders or poly-crystals commonly suffering from off-stoichiometry and parasitic phases. Here we present an approach based on Focused Ion Beam microstructuring to identify crystalline microparticles within a powder sample and to shape them into geometries suitable for resistivity measurements. Our goal is to identify the materials parameters and the intrinsic physics of crystalline materials early on in the research process, before large scale single crystals are available.

As an example, we discuss the role of two-dimensionality in layered iron-arsenide superconductors and the microscopic imprints in their order parameter structure arising from their layered nature. Our conclusions are based on out-of-plane measurements in micron-sized single crystalline structures. We will show transport evidence that an interlayer order parameter modulation in the anisotropic iron-arsenide (V₂Sr₄O₆)Fe₂As₂ leads to intrinsic Josephson junctions within the crystal unit cell; thereby showing the first example of a multi-band intrinsic Josephson system. The intrinsic junctions are identified by periodic oscillations of the flux flow voltage upon increasing a well aligned in-plane magnetic field, which are observed over an extended temperature range. This periodic flux flow modulation is a hallmark signature of Josephson vortices confined junctions within the unit-cell, and is quantitatively explained by commensurability effects between the Josephson vortex lattice and the crystal structure. These observations suggest the iron-pnictide (V₂Sr₄O₆)Fe₂As₂ to be an interesting candidate for novel phase-coherent quantum applications exploiting the multi-band nature of the material.

The more isotropic compound SmFeAs(O,F) presents an intriguing example of a weaker inter-layer modulation of the order parameter that is too subtle to form Josephson junctions but strong enough to significantly influence the vortex structure. This impartial suppression leads to a new type of “hybrid vortex” matter, sharing characteristics of both Abrikosov- and Josephson-type vortices. We show the fingerprints of these novel vortices in an unusal field- and temperature dependence of vortex commensurability oscillations and compare the results to numerical Ginzburg-Landau simulations of vortices subjected to a weak special modulation of the order parameter. These results improve our understanding of the pinning potential landscape in these materials and are of relevance for their use in high-T_c applications.

So far, YBa₂Cu₃O_7-x (YBCO) or related compounds are the most promising materials for the realization of coated conductor applications. Over the last 20 years, significant efforts have been made to develop biaxially textured templates for such conductors for long length manufacture. One major approach for the preparation of cube textured buffer layers on arbitrarily textured metal tapes is the application of ion beam assisted deposition (IBAD) or alternating beam assisted deposition (ABAD). Alternatively, highly textured metal templates are realized by the rolling assisted biaxially textured substrate (RABiTS) approach. More recently, significant efforts are devoted to enhance the current transport capability of the superconducting layers in magnetic fields by nano-engineering the microstructure. Therefore, nanometer-sized artificial pinning centers need to be implemented in the YBCO matrix with an optimized density and distribution.
We deposited YBCO layers with a thickness of 1 µm and above on buffered Ni-W RABiTS substrates as well as on ABAD-YSZ templates using pulsed laser deposition. Additionally, artificial pinning centers such as BaHfO₃ and the mixed double-perovskite Ba₂Y(Nb/Ta)O₆ were incorporated in these YBCO layers. X-ray diffraction confirmed the epitaxial growth of the superconductor on these templates as well as the biaxially oriented incorporation of the second phase additions in the YBCO matrix. Detailed TEM studies revealed the size and distribution of the nanoparticles in dependence of the growth parameters showing a combination of a-b-oriented plates and c-axis aligned nanorods. A critical current density J_c of up to more than 2 MA/cm² was determined at 77 K in self-field for 1 µm thick films. Transport measurements were performed on the grown films showing a reduced J_c anisotropy in magnetic fields for the doped samples. The influence of the deposition conditions on the distribution of artificial pinning centers and the resulting transport properties will be discussed in detail.
The authors acknowledge financial support from EUROTAPES, a collaborative project funded by the European Union's Seventh Framework Programme (FP7 / 2007 - 2013) under Grant Agreement no. 280432.

The upper limit to the critical current density J_c is determined first by vortex pinning but J_c is then frequently reduced by defects that limit the connectivity of the sample. For all cuprates, grain boundaries (GBs) tend to obstruct current whenever the grain-to-grain misorientation exceeds ~5°. Round wires of Bi-2212 are a very interesting case because they develop very high J_c when processed under pressure to make full physical connectivity and because recent work by Kametani et al. reveals a strong biaxial texture with a full-width at half-maximum (FWHM) of 15°. Unlike REBCO and Bi-2223 where a critical angle of ~5° is well established, there are no signs of weak link behavior in 2212 wires. However, 2212 can be strongly overdoped (REBCO and 2223 cannot), making the observation of high J_c without weak links with FWHM of 15° plausible. Our work involves varying the doping state of both fully and partially dense Bi-2212 round wires which should affect both the vortex pinning strength and the GB superfluid density. We observed that the change in pinning strength varied much less with doping state as compared to the magnitude of J_c and the magnetization hysteresis, implying that underdoped grain boundaries have a more significant effect on the connectivity of Bi-2212 than previously thought. The significance of finding a cuprate with a significantly larger critical angle than REBCO is immense and the wide doping range of Bi-2212 makes its study very interesting.

Engineered nanoscale defects within REBa₂Cu₃O_7-δ (REBCO) based coated conductors are of great interest for enhancing vortex-pinning, especially in high-applied magnetic fields. We have conducted extensive research to optimize vortex-pinning and enhance J_c via controlled introduction of various types of nanoscale defects ranging from simple rare-earth oxides and Ba-based perovskites to double perovskite rare-earth tantalates and niobates (Ba₂RETaO₆ and Ba₂RENbO₆). This talk will provide an overview on how density, morphology, and composition of these engineered nanoscale defects affects vortex-pinning in different temperature, field and angular regimes. Detailed microstructural and superconducting properties coated conductors with these engineered defects will be presented. It will be shown that certain nanodefect configurations that provide the best performance at high-operating temperatures also provide the optimal properties at low operating temperatures out to high-applied magnetic fields.

YBa₂Cu₃O_7-δ based coated conductors have large potential in such diverse applications as wires/cables, motors/generators, high-field coils, and superconducting permanent magnets, each of them with a certain range of temperature and magnetic field and a certain need in magnitude and isotropicity in critical current density J_c. In order to use the full potential of YBa₂Cu₃O_7-δ, it is mandatory to tailor its transport properties for the envisaged application. This is done by inserting artificial pinning centers, such as perovskites (e.g. BaZrO₃) or (mixed) double-perovskites, which precipitate as nanoparticles and/or nanorods.
We report on a study on the addition of the mixed double perovskite Ba₂Y(Nb/Ta)O₆ to YBa₂Cu₃O_7-δ thin films on SrTiO₃ single crystals prepared by pulsed laser deposition. Size, shape, density and orientation distribution of these pinning centers are analysed by X-ray diffraction and TEM. The electrical transport properties are determined in 4-point geometry in maximum-Lorentz force configuration in fields up to 9 T on bridges prepared by laser cutting.
For small deposition rates, Ba₂Y(Nb/Ta)O₆ grows as well oriented, densely distributed nanocolumns (D = 10 nm). We achieved a pinning force density of 25 GN/m³ at 77 K at the matching field of 2.3 T, which is among the highest values reported for YBa₂Cu₃O_7-x. The field dependence of the pinning force density and the anisotropy of the critical current density show a complex behavior which is explained by a matching effect of the magnetic field’s c-axis component and the superposition of up to three pinning components. The exponent N of the current-voltage characteristics (inversely proportional to the creep rate S) elucidates the depinning mechanism, changing from double-kink excitation below the matching field to pinning-potential-determined creep above.

Cuprate high temperature superconducting (HTS) cables offer powerful opportunities for increasing capacity, reliability, and efficiency of the electricity grid. HTS coils can provide an alternative to rare-earth permanent magnets used in rotary machines and generators. Increasing their current capacity in the presence of magnetic fields is critical for HTS application in generators for wind turbines, propulsion motors, magnets used in accelerators and medical imaging machines. Here, we present our recent study carried out at BNL’s Tandem Van de Graaff facility. We demonstrated a roll-to-roll irradiation process on production long length coated conductors that resulted in uniform enhancement of critical current in the 4 – 50 K at magnetic field above 1.5T using 18 MeV Au ions at a dose of 6x10¹¹ Au ions/cm².[1] The moderate ion energies used here are readily accessible with commercial electrostatic generators.

*This work was primarily supported by the US Department of Energy, Advanced Research Project Agency-Energy (ARPA-E), in collaboration with T. Ozaki, C. Zhang, L. Wu, M. W. Rupich, and V. Solovyov. T. Ozaki, C. Zhang, and L. Wu were supported by DOE Office of Science, Materials Sciences and Engineering Division.

[1] Martin Rupich, Q. Li et al, “Engineered Pinning Landscapes for Enhanced 2G Coil Wire” EUCAS 2015.

(RE)BaCuO bulk high temperature superconductors fabricated in the form of large, single grains can generate magnetic fields that are an order of magnitude higher than those achievable using conventional permanent magnets. The microstructures of these materials are known to play a key role in determining their superconducting properties, such as critical current density and resultant trapped field.
The Seeded Infiltration and Growth (SIG) process is an alternative to conventional Top Seeded Melt Growth (TSMG) for growing (RE)BCO bulk superconductors since it overcomes drawbacks such as porosity, sample shrinkage and inhomogeneity in the distribution of (RE)₂BaCuO₅ (RE-211) content across the volume of the samples, which are inherent to the TSMG process. However, due to the complexity in the growth process, it is not trivial to make single grain samples more reliably via the SIG process. Furthermore, to date the superconducting properties exhibited by infiltration grown samples are inferior compared to their counter parts made by conventional TSMG.
In the present work, we report a modified TSIG technique referred as Two-step buffer-aided Top Seeded Infiltration and Growth process. This technique has made the growth process more straightforward and very reliable. Additionally, this near-net shaping technique has enabled better control of RE-211 in the fully processed bulk superconductor, which has led to an improvement in flux pinning for practical applications. YBCO samples up to a diameter of 32 mm have been fabricated successfully by this new technique. A typical, 25 mm diameter YBCO sample has exhibited a trapped field of 0.84 T at 77 K. Details of the development of this novel fabrication technique will be presented and the results achieved will be discussed.

Current blockage at grain boundaries (GBs) has been the central problem for high temperature superconductor (HTS), demanding complicated and/or expensive processing for wire fabrications. With the drawback of T_c suppression, it is known that Ca doping can improve critical current density J_c across [001] tilt low angle (5-7°) GBs (LAGBs) in REBa₂Cu₃O_7-δ (RE123). Although previous works suggested possible scenarios, its mechanism has not been fully understood. Here we report atomic-structural and stoichiometry studies of Ca-doped Yb123 [001] tilt LAGBs by using atomic resolution analytical scanning transmission electron microscopy (STEM). By Ca doping, LAGBs can be considered as two distinct regions, the GB dislocations and channels, former of which strengthen superconductivity of the latter by providing excess hole carriers. The strained regions at GB dislocations expand ~3 times by preferential Ca substitution with Yb and Cu, attracting excess oxygen. Meanwhile local Ca content decreases by ~50% at the GB channels. Both of these suppress oxygen δ at the GB channels, producing the similar effect of local oxygen overdoping that provides more hole carriers.

To tailor or design the properties of a superconductor, thin film is a very important form of material. We have been studying the film growth of the layered iron pnictide system AEFe₂As₂ (AE = alkaline earth, henceforth 122 system) by molecular beam epitaxy (MBE). In this talk, we report our recent achievements on two topics: the first epitaxial film growth of Ca-based 122 (Ca122), and the field-effect control of carrier density of Ba-based 122 (Ba122).
Recently, Ca122 is drawing a considerable interest because of the high T_c in Ln-doped (Ln = lanthanide) single crystals reported by several groups. The reported T_c’s are even higher than 45 K and closing up the record T_c of 56 K by LnFeAs(O,F). Hence, the realization of film growth of this system is highly desirable from the view point of both fundamental physics and applications, yet the attempts had not been successful thus far. In this study, we carefully optimized the growth parameters, especially the flux ratios of the source materials. As a result, we have successfully grown epitaxial thin films of the mother compound on various substrates. As the film grown on (La,Sr)(Ta,Al)O₃ (LSAT) substrate had the best quality, we then have grown CaFe₂(As_1-xP_x)₂ films with various P contents on LSAT substrates and observed clear superconducting transitions. The phase diagram showed a difference from the report on bulk single crystal, i.e., an expansion of the superconducting phase to higher P-content was observed. Our analyses of the structural data indicate that the difference with bulk samples can be explained by the strain effect caused by the lattice mismatch with the substrate, implying that the properties can be controlled by strain-mediated tuning.
In the iron pnictide system, it is interesting that the superconducting nature is strongly affected by both structural deformation and charge carrier doping. To tailor the properties of the material, it is desirable to control these two variables independently. Aliovalent doping introduces structural change through chemical pressure without changing the carrier density. However, it is not possible to change the carrier density alone by chemical substitution, as it induces always a local structural deformation. In this context, an ion-gated transistor, which can accumulate carriers with a density 100 times larger than conventional transistors, would be a powerful tool to scrutinize the effect of changing the carrier density in the iron pnictides. Because of the chemical stability, Ba122 system is one of the most suitable candidates for this class of experiments. We successfully fabricated ion-gated transistors from P-doped Ba122 films, and performed carrier accumulation experiments both for electron doping and hole doping. The gating experiments resulted in slight decrease of T_c for both directions, indicating a new possibility of tailoring the superconducting properties in the iron pnictide systems.

Diamonds show superconductivity by doping boron heavily. The critical boron concentration is estimated to be around 3×10²⁰ cm^-3[1,2]. The first superconducting boron-doped diamond was fabricated by high pressure and high temperature (HPHT) method in 2004, and since then, many groups have challenged to make superconducting diamonds by using several methods. Our group think that Micro-wave Plasma assisted Chemical Vapor Deposition (MPCVD) is the perfect method for growing high quality heavily boron-doped diamond thin films. Using this method, we have succeeded in making Tc(offset)=10K diamonds, and some showed signs of Tc(onset)=25K [3]. In this study, we report the several properties of superconducting diamonds, especially the effect of lattice distortion and lattice relaxation on superconductivity.
Superconducting diamond thin films were synthesized on HPHT Ib single crystal diamond substrate by MPCVD method. H₂+CH₄+Tri-Metyl-Boron (TMB) gas mixture was used for growth. The Boron concentration in diamonds was controlled by gas ratio ([TMB]/ [CH₄], [CH₄]/ [Total gas]), deposition temperature, and pressure. We found lattice orientation dependence of Tc. Tc of (111), (100) and (110) superconducting diamonds rise with increasing boron concentration, then Tc of (100) and (110) diamonds saturate around 4K but only Tc of (111) diamond does not saturate and we achieved Tc(offset)=10K and boron concentration=１×10²² cm^-3. We revealed that this difference is caused by lattice distortion. In (100) diamonds, if boron atoms exist at the substitutional site, the lattice always expand in both perpendicular and horizontal directions. On the other hand, in (111) diamonds, two type of lattice expansion is considered. One type is the same as (100) and the other type is only perpendicular expansion. And our superconducting (111) diamonds showed only perpendicular lattice expansion measured by (113) reflection asymmetric Reciprocal Space Mapping (RSM). We also revealed the lattice relaxation and the relationship between critical thickness and boron concentration. Heavily boron doped diamonds with a boron concentration of 10²¹ cm^-3 have lattice constants that are 0.2-0.7% larger than that of pure diamonds. Due to this lattice mismatch, lattice strains appear and once the doped diamond film reaches a certain thickness, relaxation of the lattice strain caused by in-plane stress begins to occur. This thickness is called critical thickness. We observed that the critical thickness becomes thinner with increasing boron concentration, and the thinnest thickness was obtained when the boron concentration is １×10²² cm^-3.
We will report on other physical property on the day of presentation.

[1] YoshihikoTakano, Hiroshi Kawarada et al., APL, 85 2853 (2004)
[2] Akihiro Kawano, Hiroshi Kawarada et al., Phys. Rev. B 82, 085318 (2010)
[3] Hiroyuki Okazaki, Hiroshi Kawarada, Yoshihiko Takano et al., APL, 106, 052601 (2015)

FeSe(Te), which belongs to the Fe-based family of superconducting compounds, has the simplest crystal structure. It consists of only the continuous stacking of tetrahedral Fe(Se/Te)₄ layers arranged along the c-axis. Fe_1+yTe with crystal structure analogous to Fe_1+ySe does not show superconductivity but rather undergoes a structural transition from tetragonal to orthorhombic near 70 K. However, chemical doping at the Te-site in Fe_1+yTe system with Se or S gives rise to superconductivity. Highest T_c (~ 15 K) is observed when Se substitutes for Te in FeTe system with the composition FeTe_1-xSe_x (x = 0.50). As this compound has an extremely simple crystal structure, excellent response to external pressure and the less toxic Se in comparison to As in pnictides, it presents a better model to study the interplay between structure, superconductivity and magnetism in the Fe-based superconducting compounds. In order to study the effect of varying the lattice parameters of FeTe_0.6Se_0.4 system on its superconductivity and magnetism, we have used S to substitute at the Se-site. We have grown single crystalline samples of FeTe_0.6Se_0.4-xS_x (x = 0, 0.05 and 0.10) using self-flux technique and studied the structural, electrical transport and magnetic properties of the as-grown crystals. The single crystals obtained are plate like and appear black with shining texture. The X-ray diffraction done on the cleaved surface confirms that the plane of cleaved surface is (0 0 l). The powder X-ray diffraction done on the powdered single crystals reveals a single (tetragonal) phase with space group symmetry P4/nmm for all the samples. We have observed that the lattice parameters a and c decrease slightly with S-substitution at the Se-site in FeTe_0.6Se_0.4 due to smaller ionic radius of S as compared to Se. This is also clear from the shifting of (0 0 l) peaks to higher angles. The temperature dependence of resistivity measurements is done for all the samples. It is observed that the value of T_c (~ 14 K for x = 0) decreased with the substitution of S (~ 9 K for x = 0.10), which is also confirmed from ac-susceptibility measurements. The resistivity measurements for all the samples have also been done in the presence of magnetic fields (0 – 8 T) to estimate the upper critical field (H_c2) and irreversibility field (H_irr) values using the criteria of H_c2 = H(0.9ρ_n) and H_irr = H(0.1ρ_n). The values of H_c2(0) have been calculated by fitting the experimental data with GL-theory: H_c2(T) = H_c2(0)(1-t²)/1+t²), where t = T/T_c. The value of H_c2(0) ( ~170 T) observed is highest for x = 0. The high sensitivity of superconductivity towards structure has been observed in the present study.

Understanding the interplay between superconductivity and magnetism in iron-based superconductors is likely to provide significant insights into the elusive mechanisms responsible for high temperature superconductivity. The co-existence of magnetic order and superconductivity is a common feature of the iron-based superconductors, raising the question of whether these phases are spatially distinct or whether the same electrons are responsible for both phenomena.
In the case of the A_xFe_2−ySe₂ compounds, with T_c values of about 30K, the intrinsic phase separation is very extreme, with significant chemical and structural differences associated with the spatially distinct electronic phases. Here we will present recent results on the microstructural development of A_xFe_2−ySe₂ single crystals upon annealing, and how this influences their superconducting/magnetic properties. In addition to analytic electron microscopy studies, scanning photoemission microscopy (SPEM) has been used to investigate the differences electronic structure between the two phases, enabling direct correlation between the electronic properties and chemistry/structure of each phase. Recent work using Photoemission electron microscopy (PEEM) to carry out spatially resolved core level X-ray Absorption Spectroscopy and linear dichroism experiments will also be reported.

MgB2 superconducting bulk magnets have been intensively studied for the past five years. Since it is well known that the grain boundaries are predominant pinning centers for MgB2, the refinement of grains and/or the densification have improved the trapped field properties. Recently, a hot pressed (HPed) MgB2 bulk using the ball-milled nano-powder trapped 5.4 T at 12 K [1]. We also obtained the trapped field of 4.6 T at 14 K at the center of doubly stacked Ti-doped bulks fabricated by a hot isostatic pressing (HIPing) method [2]. The dense MgB2 bulks can also be synthesized by an infiltration method without pressing and specific furnace, contrary to the HP and HIP methods. Giunchi et al. succeeded in fabricating large and dense MgB2 bulk by a reactive liquid Mg infiltration (Mg-RLI) method [3], and trapped field of 2 T at 20 K was obtained for the bulk with 55 mm in diameter. In this paper, we prepared MgB2 bulks (about 30 mm in diameter) by the infiltration method in a home-made closed capsule and evaluated their trapped field and vortex pinning properties. The filling factor of the obtained bulk was estimated to be about 90% and the critical temperature defined at the mid-point of the magnetic transition was about 38 K. The bulk was magnetized by a field cooled magnetization under a magnetic field of 5 T. The trapped field at the center of the bulk surface was 2.4 T at 16.2 K, which was comparable with that of the HIPed bulk with the similar density and size. We discuss a potential of the MgB2 bulk fabricated by the infiltration method as the superconducting bulk magnet.
References
[1] G. Fuchs et al., Supercond. Sci. Technol. 26 (2013) 122002.
[2] T. Naito et al., Supercond. Sci. Technol. 28 (2015) 095009.
[3] G. Giunchi et al., Int. J. Mod. Phys. B 17 (2003) 453.
Acknowledgements:
This work was partly supported by JSPS KAKENHI Grant Numbers 15K04718, 15K04646.

A review about progress in fabrication, measurements and theoretical interpretation of Josephson effects in the new iron-based superconductors is given. The main properties and parameters of Josephson junctions with respect to basic understanding as well as to future applications like the product of critical Josephson current and normal resistance are discussed. Preparation and properties of different types of junctions (artificial barrier SNS, SINS or SIS junctions and bicrystal grain boundary junctions) using thin films or single crystals of iron-based superconductors as one or both electrodes are compared. Temperature and magnetic field dependencies as well as the effects of microwave irradiation are measured and compared to theoretical models. New designs will be introduced to realize phase sensitive junctions for investigation of the pairing symmetry in the iron-based superconductors.

Few-nanometer-thick films of low temperature superconducting materials have become increasingly important for sensitive photonic devices. Hot electron bolometers (HEB), superconducting nanowire single photon detectors (SNSPD), microwave kinetic inductance detectors (MKID) and transition edge sensors (TES) are all broadband, highly sensitive photo-detectors that are fabricated from thin films of low temperature superconducting materials.

Improvements to materials synthesis and exploration of materials dependent properties of these devices has so far taken a back seat to advancing the performance of individual devices through electrical design. In particular, while the performance of individual few-nanometer-thick niobium nitride (NbN) SNSPDs has produced detectors with near unity quantum efficiency and tens of picoseconds of jitter, yield of these devices remains a major issue. As optical systems demand longer nanowire devices and arrays of devices, previously ignored yield issues must be addressed. Promising results have been achieved in superconducting amorphous tungsten silicide (WSi), suggesting that yield is improved by moving from a polycrystalline material system such as NbN to an amorphous one. However, the operating temperature required for efficient operation of WSi SNSPDs (
In this work we explored reactive DC magnetron sputtering of few-nanometer-thick NbN without intentional substrate heating, as well as the performance of superconducting devices made from this material. We have found that for films with thicknesses typical of those used to fabricate SNSPDs, the addition of a large substrate bias and the ion bombardment that results during film deposition significantly reduces the film resistivity, increases the T_c and decreases the film roughness. The film surface morphology is changed significantly. This material shows promise for fabricating SNSPDs with improved yield, increased efficiency and at easy to access temperatures. Additionally, a host of fabrication methods and structures become practical when heating of the substrate is not required.

Superconducting nanowire single photon detectors (SNSPDs) have attracted much interest due to their the higher detection efficiencies, lower jitter counts and, most importantly, faster reset times allowing for operation at telecoms wavelengths. The first devices were fabricated using highly crystalline NbN as the superconductor [1]. NbN and NbTiN continue to dominate the field in terms of wide use however a new generation of devices based on amorphous superconductors such as WSi or MoSi are beginning to challenge for the highest detection efficiencies at a wide range of wavelengths [2]; their reduced superconducting energy gap results in larger hotspot formation allowing for detection of lower energy photons. The amorphous nature allows for a more homogenous film to be grown which results in a spatially uniform detection efficiency and higher internal quantum efficiency.
The conflicting growth requirements of crystalline and amorphous films provide an interesting study in to the material science required for the different growth mechanisms. Additionally, other device requirements, such as thermally unstable ion implanted waveguides or reflective layers below the detector, place restrictions on the growth parameters meaning non-deal conditions must be optimised.
In this work we present the state of the art for different approaches to SNSPD fabrication within the context of the materials used. Our work in optimising both crystalline NbN and amorphous MoSi [3] films provides an important backdrop to the observations of other research groups. With different advantages and disadvantages, a better understanding of the materials themselves is vital for continued development of SNSPDs.
[1] A.D. Semenov et al Quantum detection by current carrying superconducting film Phys. C Supercond. Its Appl. 351, 349, 2001
[2] B Baek et al Superconducting a-WxSi1−x nanowire single-photon detector with saturated internal quantum efficiency from visible to 1850 nm APL, 98 25, 251105, 2011
[3] D. Bosworth et al, Amorphous molybdenum silicon superconducting thin films, AIP Adv. 5, 087106, 2015

In the past few years, tremendous advancements have occurred in nanoscale defect engineering in REBCO coated conductors that has led to a quadrupling of critical current performance in high magnetic fields and operating temperatures of interest for various applications. REBCO tapes have been fabricated with a high density of nanoscale defects with zirconium addition up to 25 mol% with excellent properties in magnetic fields of 2 – 30 T over a temperature range of 4.2 K to 77 K. Critical current densities exceeding 20 MA/cm² have been achieved at 30 K, 3 T (field perpendicular to tape) and pinning forces over 1000 GN/m³ have been attained at 20 K. Combining these heavily-doped REBCO with thicker films, engineering current densities above 2200 A/mm² have been demonstrated at 4.2 K in 15 T field oriented perpendicular to the tape plane, which is twice and thrice respectively better than the performance levels of Nb₃Sn and Bi-2212 wires at this field. An overview of the development of high performance REBCO coated conductors for high field applications will be presented.

This work was supported the Advanced Research Projects Agency-Energy, Office of Naval Research and Argonne National Laboratory.

Sumitomo Electric has been developing the silver-sheathed Bi2223 multi-filamentary wires, DI-BSCCO (Dynamically-Innovative BSCCO). The wires have been improved various properties in response to the growing demands from the application products and projects.
DI-BSCCO wires are laminated with high-strength materials for high-field applications to endure the intense hoop stress. Such applications also need higher critical current density, especially, engineering critical current density (J_e) to make the magnets lighter and more compact. Although the thinner reinforcement materials are inevitable for high J_e, the thickness and strength are incompatible with each other. This fact urged us to explore the new potential materials for reinforcement with higher strength beyond stainless steel. Among the reviewed materials, Ni-Cr alloys have been focused on as the promising reinforcement materials.
The reinforcement with the thin (30 μm-thick) Ni-Cr alloys has considerably enhanced the mechanical properties of DI-BSCCO wires. For example, critical double bending diameter at room temperature ~ 34 mm, critical tensile stress at 77 K ~ 435 MPa, and critical tensile strain at 77 K ~ 0.5 %. The wires laminated with Ni-Cr alloys were previously called Type HT-XX (~2014) and have been officially named Type HT-NX (2015~). In this talk, the detailed performances of the currently available wires and the updated R&D activities will be shown.

The field of superconductivity in Fe-based compounds is abuzz with the search for a technique to unambiguously determine the gap symmetry and its structure. There have been many proposals in the past have been made with regard to the above and a lot of experiments done employing various techniques. Nonetheless, the quest for the irrefutable underlying superconducting mechanism seems to have hopes pinned on the quasiparticle interference (QPI) data interpretation , in a qualitative manner, from the Fourier transformed STM measurements. In this work, we show that the idea of using QPI is a probe for the gap symmetry needs to be addressed thoroughly, such
that the various aspects of the phenomena are explored from the weak coupling to the strong coupling regime. Here, we utilize the well known strong interaction Eliashberg theory as a calculation tool to probe the two regions and extract the physical meanings of the QPI for s++ and s�+- , intraband and interband transitions respectively, at both the scales in a quantitative manner. Although, the
full knowledge of the gap symmetry itself wont divulge any associated structural details of the order parameter and the mass gap, yet it has a strong bearing upon the pairing mechanism or the proverbial "bosonic gule" of high temperature superconductivity.The method of using the quasiparticle interference for classifying symmetry of the superconducting gap seems very promising and needs to be addressed extensively in order to significantly enhance the knowledge of superconducting mechanism in pnictides.

For conventional superconductors, the superconductive critical temperature can be characterized by the coupling strength, λ=NV where N is the single particle density of states and V is an attractive potential. The search for new superconductors with high T_c often involves searching for materials with enhanced N. In this presentation we explore an alternative approach that entails enhancing V. We demonstrate empirically that several disparate classes of superconductors share a common phase diagram T_c(r) where r is a coordinate characterizing the distance to the MIT. This phase diagram may be explained by explicitly incorporating the novel properties of the MIT in V. We discuss the implications of this model on HTS and how this framework can be used as a guide in the search for new superconductors.

We have demonstrated that metamaterial approach to dielectric response engineering, increases the critical temperature of a composite superconductor-dielectric metamaterial in epsilon near zero (ENZ) regime. To create such metamaterial superconductors two approaches were implemented. In the first approach, mixtures of tin and barium titanate nanoparticles of varying composition were used [1]. An increase of the critical temperature of the order of 5% compared to bulk tin has been observed for 40% volume fraction of barium titanate nanoparticles. Similar results were also obtained with compressed mixtures of tin and strontium titanate nanoparticles. In the second approach, we demonstrate the use of Al₂O₃-coated aluminium nanoparticles to form epsilon near zero (ENZ) core-shell metamaterial superconductor with a T_c that is three times that of pure aluminium [2]. IR reflectivity measurements confirm the predicted metamaterial modification of the dielectric function thus demonstrating the efficacy of the ENZ metamaterial approach to T_c engineering. The developed technology enables efficient nanofabrication of bulk aluminium-based metamaterial superconductors. These results open up numerous new possibilities of considerable Tc increase in other superconductors. This work was supported in part by NSF grant DMR-1104676.
References
1. Vera N. Smolyaninova, Bradley Yost, Kathryn Zander, M. S. Osofsky, Heungsoo Kim, Shanta Saha, R. L. Greene, and Igor I. Smolyaninov, Scientific Reports 4, 7321 (2014).
2. Vera N. Smolyaninova, Kathryn Zander, Thomas Gresock, Christopher Jensen, Joseph C. Prestigiacomo, M. S. Osofsky, and Igor I. Smolyaninov, (2015), in press.

In this presentation, the authors present a summary of some of the current state-of-the-art in modelling the magnetization of bulk superconductors, including both (RE)BCO (where RE = rare earth or Y) and MgB₂ materials. Such numerical modelling is a powerful tool to understand the physical mechanisms of bulk superconductor magnetization, to assist in interpretation of experimental results, and to predict the performance of practical bulk superconductor-based devices, which is particularly important as many superconducting applications head towards the commercialisation stage of their development in the coming years. The presentation highlights the differences between simulating the different magnetizing techniques – zero field cooling (ZFC), field cooling (FC) and pulsed field magnetization (PFM) – as well as the differences and similarities of the assumptions made for the numerical simulation related to bulk (RE)BCO and MgB₂ samples.

The implementation of YBa₂Cu₃O_7-δ coated conductors in large scale applications generally requires an improvement of the superconductors’ in-field performance. The incorporation of nanostructured secondary phases has proven to be an adequate approach to achieve flux pinning, hereby preventing the drastic decrease of the critical current density J_c at moderate-to-high magnetic fields as well as its angular dependency on the magnetic field. The most effective pinning occurs when the nanometer-sized defects are homogeneously distributed throughout the superconductor. To achieve this, we have produced nanocomposite thin films starting from preformed nanocrystals (PNC) in combination with fluorine-based chemical solution deposition. The use of PNC’s generally offers a better control of the final microstructural properties of the nanocomposite films when compared to self-assembled nanocrystal formation during YBCO processing.
In this work, small ZrO₂ and HfO₂ nanocrystals were synthesized with diameters in the range of 4 - 8 nm and a narrow size distribution. The NCs are initially capped with hydrophobic ligands, ensuring colloidal stability in apolar solvents. However, as the YBCO precursor typically provides a more polar environment, e.g., methanol, an important aspect of this research involves ligand exchange and the appropriate stabilization procedure. We are able to stabilize these nanocrystals in different types of fluorine-based YBCO precursor solutions, leading to highly stable nanocomposite precursors with long shelf-lives. Afterwards, the YBCO-PNC solutions were deposited on single crystal LaAlO₃ substrates via both spin-coating and ink-jet printing. The main focus of this research pointed to understanding the factors which control the microstructure development and the physical properties of the nanocomposite thin films. By strict optimization on both the precursor and processing level, we achieved nanocomposite thin films exhibiting J_c of 4-5 MA/cm² at 77 K in self-field as well as a much smoother decay of J_c as a function of magnetic field. This is reflected by a strong pinning force enhancement (up to 10 GN/m³ at 77K) and a reduced effective anisotropy (γ_eff ~ 3) compared to undoped YBCO films.
This newly developed approach delivers scalable and high quality superconducting films, capable of meeting the strict requirements for the successful implementation and distribution of coated conductors throughout energy market.

The recently discovered Fe-based superconductors (FBS) include many compounds with different intrinsic and extrinsic properties. In order to understand which of these compounds has potential for applications, many aspects have to be investigated. Here we will focus on the vortex pinning properties in the LnFeAs(O,F) (1111) and AEFe₂As₂ (122) phases. It has been demonstrated that Co-doped Ba122 with a T_c of only 22 K has an exceptional acceptance of high densities of secondary phases and defects [1,2,3] capable of acting as effective pinning centers with pinning force density up to 84 GN/m³ at 22.5T and 4.2 K [3]. This is the highest value ever reported in any 122 phase and it shows that the in-field performance of Ba122 widely exceeds that of Nb₃Sn, potentially making 122 of interest for applications. The 1111 phases are in principle of even greater potential because of their twice higher T_c. We discovered that single layer thin films are influenced by both intrinsic [4,5] and extrinsic pinning [5]. In Nd1111 we found two distinct and weakly interactive extrinsic pinning mechanisms whose contribution independently changes with temperature. Moreover the 1111 phase, because of the larger T_c and larger anisotropy with respect to the 122 phase, behaves as quasi two-dimensional (2D) system with intrinsic vortex pinning properties closer to the high-T_c YBCO than to the low-T_c superconductors.

Work made in collaborations between ASC-National High Magnetic Field Laboratory-Florida State University, Nagoya University, Institute for Metallic Materials – IFW, Karlsruhe Institute of Technology-Institute for Technical Physics, and University of Wisconsin-Madison.

¹ C. Tarantini et al., Appl.Phys.Lett. 96, 142510 (2010)
² C. Tarantini et al., Phys.Rev.B 86, 214504 (2012)
³ C. Tarantini et al., Sci.Rep. 4, 7305 (2014)
⁴ K. Iida, C. Tarantini et al., Sci.Rep. 3, 2139 (2013)
⁵ C. Tarantini, K. Iida, F. Kurth, J. Jaroszynski, V. Grinenko, J. Hänisch, B. Holzapfel, S. Schmidt, P.Seidel, N. Sumiya, M. Chihara, H. Ikuta, D.C. Larbalestier, in preparation.

Electron or hole doped AEFe₂As₂ (AE=Ba and Sr) and Fe(Se,Te) show high upper critical fields with low anisotropies at low temperatures that offer a unique possibility for high field magnet applications. The recent progress of powder-in-tube (PIT) technique is remarkable, resulting in high performance AEFe₂As₂ tapes. Furthermore, Co-doped BaFe₂As₂ and Fe(Se,Te) on ion beam assisted deposition (IBAD)-MgO/Y₂O₃/Hastelloy with comparable J_c to those on single crystalline substrates have been demonstrated, although the texture quality of the formers is inferior to the latters [1-3]. On the other hand, LnFeAs(O,F) (Ln=Sm, Nd) is expected to have potentially higher J_c than other Fe-based superconductors (FBS) due to the highest T_c around 55 K among the FBS. However, the level of J_c in PIT-processed SmFeAs(O,F) wire is around 4.0x10³ A/cm² even in low fields at 4.2 K [4], much lower than AEFe₂As₂ wires. A fundamental reason for the low J_c in oxypnictide may be due to higher sensitivity against the grain misorientation than other FBS, since LnFeAs(O,F) (Ln=Sm, Nd) has more anisotropic crystal structure and superconducting properties, which resembles high-T_c cuprates. Here we report on a preparation of NdFeAs(O,F) thin films on MgO-templated Hastelloy by means of molecular beam epitaxy. The oxypnictide coated conductors showed a T_c of 43 K with a self-field J_c of 7.0x10⁴ A/cm² at 5 K, more than 20 times higher than powder-in-tube processed SmFeAs(O,F) wires.

1. T. Katase et al. Appl. Phys. Lett. 98, 242510 (2011).
2. W. Si et al. Nat. Commun. 4, 1347 (2013).
3. S. Trommler et al. Supercond. Sci. Technol. 25, 084019 (2012).
4. M. Fujioka et al. Appl. Phys. Express 4, 063102 (2011).

Among transition metal nitrides, niobium nitride (NbN) stands out as a candidate for single-photon detectors (SSPDs) and superconductive radio frequency (SRF) cavities, due to its excellent superconducting properties. Despite the large number of reports on the electrical features of NbN thin films, only a few studies describe and link the structural characteristics with the superconducting properties. Typically cubic NbN (c-NbN) is deposited using dc magnetron sputtering where growth parameters, substrate or addition of a metallic element are chosen to stabilize the (Fm-3m) cubic phase, which has the highest superconducting critical temperature. Here, we report on the role of Ti in the stabilization and superconducting properties of cubic Nb_1-xTi_xN thin films, grown for the first time (to the best of our knowledge) by high temperature chemical vapour deposition (HTCVD) technique.

NbN and Nb_1-xTi_xN thin films are grown using a HTCVD reactor on different substrates – (0001) oriented single crystal Al₂O₃, (0001) AlN/Al₂O₃, and (0001) GaN/Al₂O₃. The chlorination of the Nb (99,99%) and Ti (99,99%) metallic source occurs in-situ, in a Cl₂ (99,999%) chlorination chamber. The reaction by-products with NH₃ as well as the deposition procedure are described elsewhere [1]. To access the large parameter range, the thickness of the films is varied from 90 nm down to less than 10 nm, whereas, the deposition temperature is varied from 900^oC to 1300^oC. To explore the crystal structure and surface morphology, films are characterized by means of SEM (scanning electron microscopy), XRD (X-ray diffraction) and AFM (atomic force microscope). Electrical transport measurements down to 4 K are performed in a commercial Physical Property Measurement System (PPMS).

On the structural side, it was found that for T_deposition